The Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access Network (UTRAN) is a radio network, which is also known as a third-generation (3G) mobile communication technology. Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), also referred to as Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to provide high speed packet access functionality to cope with future requirements in terms of higher data rates and improved efficiency.
An E-UTRAN of LTE typically comprises mobile terminals (MT) 150 wirelessly connected to one or several base stations 130A-C as illustrated in FIG. 1. The base stations 130A-C are shown connected to a core network (CN) 100 e.g. via a mobility management entity (MME) (not shown). In addition, the base stations 130A-C may also be connected to each other via an interface. The base stations are usually referred to as eNodeB in E-UTRAN. It should be noted that in a UTRAN a base station is referred to as a NodeB.
In LTE, a robust and fast handover is a challenging task. One reason is that in LTE, handover between two base stations implies complete cut-off of a mobile terminals connection to its serving base station, before the mobile terminal attempts to access a target base station; i.e., hard handover is used in LTE.
Another reason making a design of an efficient handover mechanism a challenging task in 3GPP LTE, is a distributed architecture of this system, which requires co-ordination between neighbouring base stations since as previously described the base stations are connected to each other. Furthermore the handover mechanism is not compliant with earlier releases of 3GPP (UMTS) where soft-handover is implemented.
Referring to FIG. 2, reference number 21 represents a serving base station which is currently serving a mobile terminal 20 and reference number 22 represents a target base station to which the mobile terminal is to be handed over. FIG. 2 illustrates a signalling flow chart during handover, as described within 3GPP LTE [2], see Appendix. In FIG. 2, signalling exchanged over an air interface between a mobile terminal (MT) 20 and the base stations 21 and 22 are shown. FIG. 2 also shows signalling exchanged via cables connecting the serving base station 21 and the target base station 22 and the target base station 22 and the MME 29.
The handover procedure starts upon triggering of an event 28 in the mobile terminal 20. For example the event 28 is triggered when the received signal strength from the target base station 22 is within a range of X dBs, e.g. 5 dB, from a signal received by the serving base station 21.
Hence, upon the event trigger 28, the mobile terminal 20 transmits a measurement report 23 to the serving base station 21. This report is a Radio Resource Control (RRC) signalling message. It is the serving base station 21 that decides to handover the mobile terminal 20 to the target base station 22. When the serving base station 21 makes a handover decision 24 it transmits a HANDOVER REQUEST message 25 to the target base station 22. This message 25 is transmitted via cable using the X2 communication protocol [2], see Appendix.
Referring back to FIG. 2, when the target base station 22 receives the HANDOVER REQUEST 25 it checks the availability of resources in the target cell served by the target base station 22. In case there are available resources, the target base station 22 transmits a response to the serving base station 21. The response is a message, HANDOVER REQUEST ACK 26. Upon reception of the HANDOVER REQUEST ACK 26, the serving base station 21 transmits a handover command 27 to the mobile terminal 20 over the air interface. The handover command 27 is also an RRC message. The serving base station 21 can start forwarding packets of the mobile terminal 20 being stored on a down link (DL) transmission buffer. Upon reception of the handover command 27, the mobile terminal 20 starts a random access procedure to the target base station 22. During the random access procedure, the mobile terminal 20 tries to synchronize with the target base station 22. Upon successful random access to the target base station 22, the mobile terminal 20 gets an allocation grant from the target base station 22. Upon reception of this grant, the mobile terminal 20 transmits a HANDOVER CONFIRM message 30 to the target base station 22 now acting as a new serving base station 22. After reception of the HANDOVER CONFIRM message 30, the new serving base station 22 transmits a HANDOVER COMPLETE message 31 to the mobility management entity 29 (MME). The HANDOVER COMPLETE message 31 is transmitted via a S1 interface built on a cable connecting the new serving base station 22 with the MME 29. A reception of this message 31 at the MME 29 triggers a switching of the path for data packets for the mobile terminal 20 being handed over. Following to the switching of paths, the MME 29 notifies the new serving base station 22 of this path switching. This notification is done with aid of a message HANDOVER COMPLETE ACK 32. The new serving base station 22 notifies the previous serving base station 21 by transmitting a message, RELEASE RESOURCES 33. This message 33 is transmitted via X2 and it signals an end of the handover procedure.
As previously described the handover command is transmitted over the air interface. The handover command comprises among others the identity of the target base station and it is the first message transmitted from the network to the mobile terminal during the handover procedure. As well known signalling messages exchanged over the air interface are most susceptible to losses. This handover command message is therefore important for the success of the handover procedure. In order to increase robustness of the whole handover procedure, the handover command is usually transmitted with very low error rate. The handover command also includes a list of parameters to be used in the target cell, e.g. parameters for radio bearer establishment in the target cell as described in [1], [2] and [3], see Appendix. In LTE, it is suggested to use additional information in the handover command, such as an identity of dedicated preambles the mobile terminal can apply in the target cell as described in [3], §5.3.4.2, see Appendix. Thus the list of parameters can be quite long. Hence, the transmission of this handover command message can require a high bit rate for the transmission of the handover command to the mobile terminal. In case the mobile terminal when receiving the handover command is located far from its serving base station or in not so favourable radio conditions it might not be able to bear the data rate which is required for transmission of the handover command.
There is therefore a need for a solution for increasing the robustness for transmission of the handover Command.